The present invention relates to a heat transfer sheet and, more particularly, to a heat transfer sheet which is useful for heat transfer systems using sublimable (or thermally migrating) dyes. The dye layer of the heat transfer sheet is easily releasable from an associated image-receiving sheet at the time of heat transfer. The heat transfer sheet provides enables a monochromic or full-color image excelling not only in image density but also in light fastness. The present invention also relates to a heat transfer process.
As replacements to generally available typographic and printing techniques, ink jet, heat transfer and other systems have been developed to provide excellent monochromic or full-color images in a simple and fast manner. The most excellent of all is the so-called sublimation heat transfer system making use of a sublimable dye, which provides a full-color image having an improved gradation or gray scale and comparable to a color photograph.
In general, a heat transfer sheet used with the sublimation type of transfer system typically includes a substrate film such as a polyester film, which is provided on one side with a dye layer containing a sublimable dye and on the other side with a heat-resistant layer for preventing it from sticking to a thermal head.
The dye layer of such a heat transfer sheet is overlaid on an associated image-receiving material having an image-receiving layer formed of polyester or other resin, and heat is applied to the heat transfer sheet from its backside in an imagewise manner, thereby causing migration of the dye through the dye layer to migrate onto the image-receiving sheet to form the desired image.
With the above-mentioned heat transfer system wherein a very quick heat transfer is needed, it is required to operate a thermal head at elevated temperatures, because heating by the thermal head must occur within a very short span of time (of the order of msec.). Increasing the thermal head temperature, however, results in a binder forming part of the dye layer being so softened that it sticks to the image-receiving sheet, leaving the heat transfer sheet bonded to the image-receiving sheet or, in the worst case, gives rise to a so-called unusual transfer problem in which the dye layer separates off and passes immediately onto the surface of the image-receiving sheet during releasing.
In order to provide a solution to the aforesaid problem, it has been proposed to add a slip agent and/or a releasant to the dye layers of image-receiving sheets (see U.S. Pat. No. 4,740,496 specification). However, this method sustains a drop of dye receptivity, and makes it difficult to laminate and bond a surface protecting layer such as a transparent film onto the resulting image, if it is needed.
In order to solve such a problem, it has been proposed to incorporate a slip agent and/or a releasant in the dye layers of heat transfer sheets without adding them to the dye-receiving layers of associated image-receiving sheets or with, if added, reducing their amount. With this method, however, substrate films tend to repel a dye layer forming-coating solution during the formation of dye layers, rendering it difficult to make them uniform and hence presenting a color shading problem to the resulting images. Moreover, the formed dye layers have such poor adhesion to the substrate films that the so-called unusual transfer is likely to occur, thus making the dye layers themselves separate off and pass onto the associated image-receiving sheets.
It is, therefore, a primary object of this invention to provide a heat transfer sheet enabling an image of better quality to be given without offering such problems as above mentioned.
On the other hand, there have heretofore been known various heat transfer techniques inclusive of a sublimation heat transfer technique in which a sublimable dye is carried on a substrate sheet, e.g., paper as a recording medium to form a heat transfer sheet. The heat transfer sheet is then overlaid on an image-receiving material, which is dyeable with the sublimable dye, e.g. an image-receiving sheet obtained by forming a dye-receiving layer on paper, plastic film or the like, thereby making various full-color images thereon.
The heating means used for this purpose is a printer's thermal head which can transfer a number of color dots of three or four colors onto the image-receiving material by very quick heating, thereby reconstructing a full-color image representation of the original image with the multicolor dots.
The thus formed image is very clear and excels in transparency due to the coloring material used being a dye, so that it can be improved in terms of the reproducibility of neutral tints and gray scale. Thus, it is possible to form a high-quality image equivalent to an image achieved by conventional offset or gravure printing and comparable to a full-color photographic image.
However, a problem with that heat transfer technique is that the resulting images are generally so inferior in light fastness to pigmentated images that they fade away or discolor prematurely when exposed directly to sunlight. Another problem is that even when they are placed in places upon which light does not strike, e.g. placed indoors or put in files or books, they tend to discolor or fade away. It has been know as a partial solution to such problems in connection with light fastness and fading in the dark by adding ultraviolet absorbers or antioxidants to the dye-receiving layers of image-receiving sheets.
With the sublimation type of heat transfer technique wherein the antioxidant, etc. are distributed uniformly over the dye-receiving layer while a large part of the dye transferred is present in the vicinity of the surface of the dye-receiving layer, however, it is impossible to provide efficient protection to the dye and so prevent discoloration and fading sufficiently. Thus, a technique enabling the dye transferred to be effectively protected by antioxidants, etc. has been in great demand.
In order to eliminate such a problem, the inventors have already proposed to incorporate an ultraviolet absorber, etc. in the dye layer of a heat transfer sheet, rather than in an image-receiving layer, and transferring the ultraviolet absorber, etc. onto an imaging region simultaneously with the transfer of the dye (see Japanese Patent Application No. Sho. 63-290101 specification).
Although such a method has been found to have some effects, however, it is not always well fit for forming a color image by repeating a plurality of transfer cycles at the same region of an image-receiving sheet. The reason for this would be that the ultraviolet absorber, etc. transferred with the dye at an initial or early stage of transfer is so caused to penetrate deeply through the dye-receiving layer by heating at a later stage of transfer that their concentration can become insufficient on the surface of the dye-receiving layer on which an actual image is to be formed.
It is, therefore, another object of this invention to provide a heat transfer sheet or process best suited for use with the heat transfer technique making use of sublimable dyes, which can provide clear images of sufficient density and having far more improved fastness properties, esp. light fastness and resistance to fading in dark places.